Chapter 3
PHOTOSYNTHESIS AND CARBON
PARTITIONING / SOURCE-SINK
RELATIONSHIPS
William T. Pettigrew
USDA-ARS, Crop Production Systems Research Unit, Stoneville, MS
INTRODUCTION
Photosynthesis is one of the principle biochemical processes underpinning plant growth
and development. Because of its basic nature, it is intimately involved with reproductive
growth and determining crop yields. Photosynthesis of a crop canopy can be broken down
into three components: 1) leaf area development, 2) photosynthetic rate per leaf area, and
3) partitioning assimilates between vegetative and reproductive growth, or source-to-sink
relationships (Krieg, 1983). The leaf surface area intercepts the solar radiation and allows
for the photosynthetic conversion of that radiant energy into chemical energy. This production of chemical energy and the subsequent use of that chemical energy use to fix CO2 into
photosynthetic carbon assimilates constitutes the source side of yield development. The
fruiting buds, flowers, and fruit development constitute the reproductive sink side of the
yield equation, although other vegetative growing points can operate as secondary sinks.
This interplay between the vegetative source and the reproductive sink can influence crop
photosynthesis because the capacity for carbon assimilation can be somewhat regulated
by the utilization of those photoassimilates in many crops (Krieg, 1983). The scope of this
review chapter is to examine the intimate but complex relationship cotton photosynthesis
(source) has with flowering and yield development (sink). The complexity occurs because
major interactions change as the boll load increases with day length, temperature, and water
availability decreasing as the season progress.
LEAF PHYSIOLOGY
The physiological changes a cotton leaf undergoes as it unfurls and expands have been well
documented. Similar patterns of peak CO2-exchange rates (CER) during leaf development
were reported by Constable and Rawson (1980a) with the peak CER occurring between 13
to 15 days after leaf unfurling, and Wullschleger and Oosterhuis (1990a) who reported peak
CER between 16 to 20 days after unfurling. This peak leaf CER occurred just before the leaf
had become fully expanded (Constable and Rawson, 1980a). Both studies demonstrated that
once this peak CER was achieved, it was then sustained for a few days before gradually declining as the leaf aged. Pettigrew and Vaughn (1998) also demonstrated that peak chlorophyll
fluorescence variable to maximum ratios (Fv/Fm), an estimate of the photosystem two (PS II)
26Pettigrew
activity, and leaf chlorophyll concentrations occurred during a similar time frame to that seen
for the occurrence of peak CER. The slight differences in the timing of the events may have
to do with the fact that field plants were utilized by Wullschleger and Oosterhuis (1990a) and
Pettigrew and Vaughn (1998), while Constable and Rawson (1980a) utilized plants grown in
the controlled environment of a greenhouse. The use of different cotton genotypes in these
various studies could also somewhat contribute to these subtle timing differences reported.
These observations have led to photosynthetic measurements in most subsequent studies generally being collected on the youngest fully expanded leaf of cotton plant to hit the period
when the peak CER was occurring. Many researchers have generally wanted to hit the period
of peak leaf CER with their photosynthetic measurement because they are often working
under the assumption that any differences they were wanting to detect with their leaf CER
measurements would be maximized during the period of peak CER. Measuring the youngest
fully expanded leaf would also minimize the confounding factor of leaf age when assessing
treatment effects on CER.
CER, LEAF AGE, AND GENETICS
The peak CER is not only determined by leaf age but also by genetics of the cotton. ElSharkawy et al. (1965) reported leaf photosynthetic differences among numerous species of
Gossypium. Beyond these species differences, a next logical place to look for photosynthetic
variation is within a species, and particularly, among genotypes that possess different types
of leaves. Across multiple studies leaf-type isolines of okra and super okra have exhibited
increased lobing and reduced area of the leaves compared to the normal leaf-type (Wells et al.,
1986), but they have not shown consistent photosynthetic differences. Leaf-type isolines did
not differ in 14CO2 fixation (Kerby et al., 1980; Karami et al., 1980) or canopy CER when that
photosynthetic rate was expressed on a leaf area basis (Pegelow et al., 1977). However, there
was an overall trend in these studies for the okra- and super okra- lines to have numerically
higher photosynthetic rates. Wells et al., (1986) reported that super okra and okra leaf-type
isolines had reduced integrated canopy photosynthesis when expressed on a ground area basis
due to their reduced overall leaf area production and canopy light interception when compared to the normal leaf-type isoline. Peng and Krieg (1991) also reported reduced late season
canopy photosynthesis on a ground area basis for okra leaf cotton, but in contrast, they reported elevated canopy photosynthesis when expressed on a leaf area basis for okra leaf lines
compared to normal leaf. Perry et al. (1983) also reported both okra and super okra leaf-types
to have greater leaf CER than normal leaf-type. Similarly, Pettigrew et al. (1993b) found that
the leaf CER of an okra leaf-type isoline was 22% greater than that of the normal leaf-type
isoline counterpart. The leaf CER of super okra leaf-type isoline was also 24% greater than
the normal leaf-type. Furthermore, Pettigrew et al. (1993b) reported the okra leaf-type line
to have greater specific leaf weights (SLW), thicker leaves, and increased leaf chlorophyll
concentrations than the normal leaf isoline. These aspects led them to conclude that the okra
leaf-type isoline had a greater concentration of the photosynthetic apparatus per unit leaf area
than the normal leaf-type isoline.
PHOTOSYNTHESIS AND CARBON PARTITIONING / SOURCE-SINK RELATIONSHIPS
27
VARIATION IN PHOTOSYNTHESIS IN
NORMAL LEAF-TYPE GERMPLASM
Genetic variation in photosynthesis can also exist within the normal leaf-type germplasm
cotton lines. Rosenthal and Gerik (1991) reported that Upland (Gossypium hirsutum L.) cultivar Acala SJ-2 had a higher radiation use efficiency than either DPL 50 or Tamcot CD3H.
Normal leaf-type genotypes that differed in their source-to-sink ratio were also found to differ
in leaf CER by Quisenberry et al. (1994). They reported that leaf photosynthesis increased
as the source-to-sink ratio was decreased. Furthermore, using a collection of 18 diverse normal leaf-type genotypes, Pettigrew and Meredith (1994) documented significant genotypic
variation in leaf CER. Pettigrew and Meredith (1994) also observed a significant positive
correlation between leaf CER and leaf chlorophyll concentration (0.768) and SLW (0.568).
Beyond the hirsutum material, genetic variation in leaf CER also exists in the Pima (Gossypium barbadense) germplasm. Cornish et al. (1991) demonstrated that modern Pima lines
had higher CER and increased stomatal conductance (gs) than older, more primitive lines.
They attributed the yield increases seen with the modern lines to the increased CER and gs. In
follow-up studies, Radin et al. (1994), Lu et al. (1994), and Lu and Zeiger (1994) indicated
that yield improvements in modern Pima lines were associated with improved heat tolerance
due to superior gs and smaller leaf size.
VARIABILITY IN CER MEASUREMENTS
Another factor that can cause variability among leaf CER measurements is the decline in photosynthesis when measured on leaves during the afternoon compared to morning photosynthetic
rates, even when measured under comparable sunlight conditions. A handful of studies have
documented lower afternoon photosynthetic rates in both Upland (Pettigrew, 2004; Pettigrew
and Turley, 1998; Pettigrew et al., 1990) and Pima (Cornish et al., 1991) cotton. Physiological
reasons for this afternoon decline are still being debated. Processes that have been implicated in
this afternoon decline for various plant species include: 1) high temperature stress (Baldocchi et
al., 1981; Perry et al., 1983), 2) photoinhibition after exposure to the intense solar radiation encountered at solar noon resulting in damage to the photosynthetic apparatus (Powles, 1984), 3) a
“down-regulation” of the photosynthesis in response to intense light conditions without damaging the photosynthetic structure by dissipating the excess absorbed photons as heat (Baker and
Ort (1992), 4) end-product inhibition of the photosynthetic process due to the buildup of large
carbohydrate levels during the afternoon hours (Nafziger and Koller, 1976; Peet and Kramer,
1980), 5) stomatal closure in response to an increasing H2O vapor pressure deficit in afternoon
hours (Bunce, 1982, 1983; Farquhar et al., 1980; Pettigrew et at., 1990), and 6) nonstomatal
photosynthetic inhibition caused by transient and localized water-deficit stress in response to
high afternoon transpiration demand (Sharkey, 1984). The complexity of this phenomenon dictate that any or all of the aforementioned processes could be contributing to the afternoon photosynthetic decline observed at any particular time. Furthermore, when the photosynthetic decline
actually begins during the day and the degree to which it inhibits photosynthesis is difficult to
28Pettigrew
predict. For instance, this phenomenon has been observed to be more pronounced under moisture deficit conditions than under well-watered conditions (Pettigrew, 2004). Nonetheless, this
afternoon photosynthetic decline must be taken into account whenever making photosynthetic
measurements in any study. Researchers would be wise to complete their CER measurements
prior to solar noon on most days to avoid this confounding effect.
LEAF CER AND YIELD
Even though theoretically there is an obvious connection between photosynthesis and reproductive growth, it has not always been easy to demonstrate the connection between leaf CER and yield
production. Part of the problem lies in the fact that one would be using an individual leaf measurement to mimic a canopy phenomenon. Another problem is that you would be using an instantaneous measurement to mimic what happens during the entire growing season (Elmore, 1980).
The perennial nature of cotton further complicates the issue because there has to be appropriate
partitioning of the increased photosynthate into reproductive growth rather than vegetative growth
to impact yield. Wells and Meredith (1984) were able to demonstrate that improved reproductive
partitioning was responsible for much of the yield improvements in newer cotton varieties. Confirmation of this photosynthesis-yield relationship has also come from a series of source-to-sink
manipulation studies (Pettigrew, 1994). Increasing the amount of sunlight intercepted by canopy
leaves through either opening the canopy by pulling back adjacent rows or by placing reflective strips between the rows increased the lint yield by 17% and 6%, respectively. Furthermore,
covering the plants with 30% shade cloth reduced the yield by 20%. Eaton and Ergle (1954) and
Guinn (1974) also noted that lowering the light intensity resulted in increased boll and square
shedding and also lower yields. Shading not only reduced yield but also reduced the fiber strength
and micronaire of the fiber compared to the fiber produced under full sunlight conditions (Eaton
and Ergle, 1954; Pettigrew, 1995). Increasing the photoassimilate supply by growing the cotton
plants under elevated CO2 conditions also has been shown to increase yields (Krizek, 1986). So
yields can be increased whenever the total pool of photosynthetic assimilates is increased, either
by increasing the amount of solar radiation that is intercepted by the canopy or by elevating the
level of CO2 the plants are exposed to during growth. By averaging the leaf CER over multiple
measurements during the boll development period, Pettigrew and Meredith (1994) were able to
demonstrate a direct connection between leaf CER and yield development for a diverse group of
18 genotypes. While increased photosynthesis can be a component of yield increases, it does not,
in and of itself, guarantee a yield increase. The extra photo-assimilates produced with elevated
photosynthesis must be directed to the reproductive structures rather than any of the potentially
competing vegetative sinks, such as roots, stems, branches, or new leaf growth.
SINK STRENGTH AND PHOTOSYNTHETIC PERFORMANCE
Documenting the effect that sink strength can exert upon photosynthetic performance for cotton
is a complex issue. This relationship is generally defined as a function of the source activity (photosynthesizing area × photosynthetic rate) and sink activity (number of actively growing sinks X
the dry matter incorporation rate) (Krieg, 1983). It is simple to believe that as the strength of the to-
PHOTOSYNTHESIS AND CARBON PARTITIONING / SOURCE-SINK RELATIONSHIPS
29
tal sink activity increases that there might also be a concurrent increase in the total source activity
to feed the growing sink demand. Finding evidence to support this assumption has proven difficult.
The results from multiple studies can be confusing to interpret and at times appear contradictory
in nature. On the one hand, Quisenberry et al. (1994) reported that single-leaf photosynthesis
increased as the source-to-sink ratio decreased, suggesting that the lower amount of source tissue
was having to increase production to meet an increased sink demand. The altered source-to-sink
ratios in the Quisenberry et al. (1994) study were accomplished with genotypes of maturity ranging from very early to very late and by also including a photoperiod sensitive line that would not
flower during the growing season at that latitude. In contrast to Quisenberry et al. (1994), when
fruit removal was utilized to increase the source-to-sink ratio, either leaf photosynthesis was unaffected (Pettigrew et al., 1993a) or the radiation use efficiency was increased (Sadras, 1996). Using
insect predation to implement their fruit loss, Holman and Oosterhuis (1999) reported the both
leaf and canopy CER were increased when the plants suffered damage from tarnished plant bugs
(Lygus lineolaris Palisot de Beauvois) and bollworms (Helicoverpa zea Boddic). They concluded
that the improved canopy CER was due to greater light penetration into the canopy also produced
by the insect infestation. Growth regulators can also be used to manipulate the source and sink
relationships. Gwathmey and Clement (2010) reported that the growth regulator mepiquat chloride reduced leaf area per plant, while also increasing the number of bolls per leaf area. Reduced
leaf area development from mepiquat chloride application was also reported by Hodges et al.
(1991), but they also found higher gross photosynthesis for the mepiquat chloride-treated plants.
Results from Hodges et al. (1991) tend to lend support to the Quisenberry et al. (1994) findings of
increased photosynthesis with decreased source-to-sink ratios. It was primarily a reduction of the
source material that decreased the source-to-sink ratios in the Quisenberry et al. (1994) work, because the sink size (boll weight plant-1) was not consistently affected by the mepiquat treatments.
SYNCHRONIZATION OF PHOTOSYNTHESIS
AND ASSIMILATE DEMAND
One of the reasons for the inconsistency in describing the relationship between photosynthesis and
sink size or strength is that the temporal pattern of photosynthetic production for an individual leaf is
not totally synchronized with the development and assimilate demand patterns of the nearby fruiting
structure. Constable and Rawson (1980a) demonstrated the peak photosynthesis for a main-stem
leaf occurred several days prior to anthesis of a 1st position bloom on the adjacent sympodial branch.
During the filling and development of the boll, the photosynthesis of that leaf had already decreased
substantially. The subtending leaf to that flower was in better synchronization as its peak photosynthesis occurred near anthesis, however, but it was not able to supply all the carbon needs of that
developing boll by itself (Ashley, 1972; Constable and Rawson, 1980b). Utilizing a carbon budget
model, (Constable and Rawson, 1980b) were able to calculate that considerable remobilization and
movement of carbon between nodes and into and out of storage pools was needed to support the developing boll load. Wullschleger and Oosterhuis (1990b) confirmed this out of step timing between
leaf photosynthesis and boll demand for assimilates with field grown plants. Constable and Rawson
(1980b) further predicted that as the cotton canopy approaches cutout, a period of slowing vegeta-
30Pettigrew
tive growth occurs due to increased assimilate demand from the developing bolls, thus, the overall
canopy photosynthesis would decline due to the aging and declining photosynthetic capacity of the
individual leaves. This prediction was confirmed by the research of Wells et al. (1986) and Peng and
Krieg (1991) who demonstrated that the canopy photosynthesis declined as the canopy aged. Unfortunately, this declining canopy photosynthesis is occurring during a period of greatest assimilate
demand from the developing boll load. These results further confirm that the lack of synchronicity
between assimilate demand of an individual developing fruit and the assimilate production from the
nearby leaves ultimately causes the demand for photoassimilates by the developing boll load to be
out of phase with the level of assimilate production by the cotton canopy.
REASONS FOR LACK OF SYNCHRONY
Part of the reason for the lack of synchrony between leaf photosynthetic potential and boll
carbon assimilate demand is that the plant essentially cannibalizes its leaves by remobilizing its
N-based components to feed the N needs of the growing bolls. Wells (1988) was able to clearly
demonstrate this property by showing that, for all but the upper most leaves, the concentrations
of chlorophyll, soluble protein, and ribulose bisphosphate carboxylase/oxygenase (Rubisco) decreased as the leaves advanced in age. Furthermore, he also demonstrated that leaves emerging
during vegetative growth had higher levels of the N components when the leaves first reached
full expansion than leaves that emerged during fruit development, when the reproductive sink for
N assimilates is growing. By assessing the photosynthetic performance on leaves of plants from
different planting dates on a mid-August measurement date, Pettigrew et al. (2000) were able
to demonstrate that leaves from early planted plants had reduced leaf CER, soluble protein, and
Rubisco activity compared to leaves from late planted cotton on that date. In contrast, they found
that neither leaf chlorophyll concentration nor the chlorophyll fluorescence variable to maximum
ratio (Fv/Fm) were altered by planting date. They concluded that N remobilization from the leaves
to feed the developing boll load initially targets the proteins in the carbon fixation half of the
photosynthetic equation and then later goes after the proteins and chlorophyll involved in light
harvesting and the conversion of that sunlight into chemical energy.
IDIOSYNCRASIES OF PHOTOSYNTHESIS
Many of the idiosyncrasies intertwined with photosynthesis and crop yield in cotton are related to the fact that cotton is a perennial crop that is grown as an annual. While this perennial
nature and indeterminate flowering pattern provides cotton with some flexibility in enduring
short-term unfavorable environmental conditions, it also leads to the out of sync fruit development pattern with regards to photosynthetic production and further complicates the defoliation and harvesting process. Maintaining crop canopy photosynthesis for a longer duration
might be a desirable pursuit because it could help to increase overall yield production since
the level of photosynthetic production would be better timed to fulfill the current reproductive
demand. Additional N fertilization has been suggested as a possible venue to maintain canopy
photosynthesis because the remobilization of N out of the leaves to support the developing boll
load was identified as a component involved in the photosynthetic decline observed late in the
PHOTOSYNTHESIS AND CARBON PARTITIONING / SOURCE-SINK RELATIONSHIPS
31
growing season. Bondada et al. (1996) were able to demonstrate higher rates of N fertilization
increased canopy photosynthesis, delayed cutout, extended the duration of the cotton canopy
and increased yields. Unfortunately, N fertilization in cotton can be a complex issue. In the Mississippi Delta, for instance, fertilization rates above 112 kg N ha-1 rarely elicit a yield response,
but in California with its higher yield potential rates, higher than 112 kg N ha-1, often produce
yield increases. The nitrogen requirement for optimal yields is complicated because of the indeterminate growth habit of cotton and the complexity of N cycling in the soil. (Gerik et al.,
1998). While higher N fertilization may extend the duration of cotton canopy photosynthesis, it
can also have negative consequences because it can create a lusher canopy that is more attractive
to insects and cause complications for the defoliation process. Clearly any bolls lost to insect
predation would create a disconnect between the higher canopy photosynthesis after additional
N fertilization and yield production.
SUMMARY
Logically it would seem to be beneficial if cotton’s photosynthetic production was better synchronized the with yield development. At the present time, however, there doesn’t appear to be
any obvious production technique to improve the timing of photosynthesis and yield production.
The perennial aspect of cotton creates additional assimilate sinks and storage pools to ensure
sufficient reserves will be available for new growth during subsequent seasons if the crop is not
cultured as an annual and dies in the off season. This lack of synchrony between photosynthetic
source production and reproductive sink demand creates the need for some assimilates from
these secondary sinks and storage pools to be remobilized to support the growing fruit load.
Because of this partial dependence on remobilized assimilate, a timing / distribution bottleneck
could potentially occur in the assimilate supply / demand function during critical phases of
reproductive growth and theoretically limit overall yield production. Conversion of cotton from
a perennial plant to an annual plant could theoretically minimize the size or totally eliminate
some of these secondary sinks and storage pools, and in the process free up some of those assimilates for further reproductive growth. There would still have to be adequate partitioning
of these “freed-up” assimilates to reproductive growth to see any improvement on the yield
front. Perhaps in the future, our friends in molecular biology and plant genomics could devote
a portion of their efforts toward producing a truly annual cotton plant for the physiologists and
agronomists to utilize.
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